The use of optical fibers and optical fiber strands in combination with light absorbing dyes for chemical analytical determinations has undergone rapid development, particularly within the last decade. The use of optical fibers for such purposes and techniques is described by Milanovich et al., “Novel Optical Fiber Techniques For Medical Application”, Proceedings of the SPIE 28th Annual International Technical Symposium On Optics and Electro-Optics, Volume 494, 1980; Seitz, W. R., “Chemical Sensors Based On Immobilized Indicators and Fiber Optics” in C.R.C. Critical Reviews In Analytical Chemistry, Vol. 19, 1988, pp. 135-173; Wolfbeis, O. S., “Fiber Optical Fluorosensors In Analytical Chemistry” in Molecular Luminescence Spectroscopy, Methods and Applications (S. G. Schulman, editor), Wiley & Sons, New York (1988); Angel, S. M., Spectroscopy 2(4):38 (1987); Walt, et al., “Chemical Sensors and Microinstrumentation”, ACS Symposium Series, Vol. 403, 1989, p. 252, and Wolfbeis, O. S., Fiber Optic Chemical Sensors, Ed. CRC Press, Boca Raton, Fla., 1991, 2nd Volume.
When using an optical fiber in an in vitro/in vivo sensor, one or more light absorbing dyes are located near its distal end. Typically, light from an appropriate source is used to illuminate the dyes through the fiber's proximal end. The light propagates along the length of the optical fiber; and a portion of this propagated light exits the distal end and is absorbed by the dyes. The light absorbing dye may or may not be immobilized; may or may not be directly attached to the optical fiber itself; may or may not be suspended in a fluid sample containing one or more analytes of interest; and may or may not be retainable for subsequent use in a second optical determination.
Once the light has been absorbed by the dye, some light of varying wavelength and intensity returns, conveyed through either the same fiber or collection fiber(s) to a detection system where it is observed and measured. The interactions between the light conveyed by the optical fiber and the properties of the light absorbing dye provide an optical basis for both qualitative and quantitative determinations.
Of the many different classes of light absorbing dyes which conventionally are employed with bundles of fiber strands and optical fibers for different analytical purposes are those more common compositions that emit light after absorption termed “fluorophores” and those which absorb light and internally convert the absorbed light to heat, rather than emit it as light, termed “chromophores.”
Fluorescence is a physical phenomenon based upon the ability of some molecules to absorb light (photons) at specified wavelengths and then emit light of a longer wavelength and at a lower energy. Substances able to fluoresce share a number of common characteristics: the ability to absorb light energy at one wavelength; reach an excited energy state; and subsequently emit light at another light wavelength. The absorption and fluorescence emission spectra are individual for each fluorophore and are often graphically represented as two separate curves that are slightly overlapping. The same fluorescence emission spectrum is generally observed irrespective of the wavelength of the exciting light and, accordingly, the wavelength and energy of the exciting light may be varied within limits; but the light emitted by the fluorophore will always provide the same emission spectrum. Finally, the strength of the fluorescence signal may be measured as the quantum yield of light emitted. The fluorescence quantum yield is the ratio of the number of photons emitted in comparison to the number of photons initially absorbed by the fluorophore. For more detailed information regarding each of these characteristics, the following references are recommended: Lakowicz, J. R., Principles of Fluorescence Spectroscopy, Plenum Press, New York, 1983; Freifelder, D., Physical Biochemistry, second edition, W.H. Freeman and Company, New York, 1982; “Molecular Luminescence Spectroscopy Methods and Applications: Part I” (S. G. Schulman, editor) in Chemical Analysis, vol. 77, Wiley & Sons, Inc., 1985; The Theory of Luminescence, Stepanov and Gribkovskii, Iliffe Books, Ltd., London, 1968.
Many of the recent improvements employing optical fiber sensors in both qualitative and quantitative analytical determinations concern the desirability of depositing and/or immobilizing various light absorbing dyes at the distal end of the optical fiber. In this manner, a variety of different optical fiber chemical sensors and methods have been reported for specific analytical determinations and applications such as pH measurement, oxygen detection, and carbon dioxide analyses. These developments are exemplified by the following publications: Freeman, et al., Anal. Chem. 53:98 (1983); Lippitsch et al., Anal. Chem. Acta. 205:1, (1988); Wolfbeis et al., Anal. Chem. 60:2028 (1988); Jordan, et al., Anal. Chem. 59:437 (1987); Lubbers et al., Sens. Actuators 1983; Munkholm et al., Talanta 35:109 (1988); Munkholm et al., Anal. Chem. 58: 1427 (1986); Seitz, W. R., Anal. Chem. 56: 16A-34A (1984); Peterson, et al., Anal. Chem. 52:864 (1980): Saari, et al., Anal. Chem. 54:821 (1982); Saari, et al., Anal. Chem. 55:667 (1983); Zhujun et al., Anal. Chem. Acta. 160:47 (1984); Schwab, et al., Anal. Chem. 56:2199 (1984); Wolfbeis, O. S., “Fiber Optic Chemical Sensors”, Ed CRC Press, Boca Raton, Fla., 1991, 2nd Volume; and Pantano, P., Walt, D. R., Anal. Chem., 481A-487A, Vol. 67, (1995).
More recently, fiber optic sensors have been constructed that permit the use of multiple dyes with a single, discrete fiber optic bundle. U.S. Pat. Nos. 5,244,636 and 5,250,264 to Walt, et al. disclose systems for affixing multiple, different dyes on the distal end of the bundle, the teachings of each of these patents being incorporated herein by this reference. The disclosed configurations enable separate optical fibers of the bundle to optically access individual dyes. This avoids the problem of deconvolving the separate signals in the returning light from each dye, which arises when the signals from two or more dyes are combined, each dye being sensitive to a different analyte, and there is significant overlap in the dyes' emission spectra.
Most recently, fiber optic sensors have been employed in arrays of semi-selective chemical sensors and pattern recognition schemes to discriminate and quantify odors. Such approaches have been useful in implementing the principles of biological olfaction in the design of sensing devices or systems. In this field of biomimetry, various technologies have been applied to the sensor transduction mechanism. For example, surface acoustic wave, conducting polymer, metal oxide sensor field-effect transistor (MOSFET), piezo-electric, and quartz crystal microbalance sensor arrays have been pursued.
While such technologies provide inventive approaches utilizing a variety of physical and chemical phenomena to odor sensing, there are a number of limitations to these methods which restrict the efficacy of such devices. Firstly, element-to-element reproducibility both within a single array and between sensor arrays is typically unsatisfactory and thus requires recalibration and network retraining from sensor to sensor. Secondly, most of these methods have a relatively slow response time, frequently requiring several minutes to respond to the presence of an odor. Thirdly, such methods have relatively high detection limits and low sensitivity, typically not functioning at odor levels below 10 ppm. Fourthly, devices which embody such technologies typically require a relatively large inherent size, thereby restricting miniaturization of the sensor array for use in remote sensing applications. Finally, construction of multi-sensor arrays by these methods is complex and involves expensive and tedious preparation and placement of individual sensors within a well-defined array.
Most recently, many of these shortcomings have been overcome through the application of fiber optic sensor arrays in an artificial nose sensor device and system. U.S. Pat. Nos. 5,320,814 and 5,512,490 to Walt, et al., the teachings of each of these patents being incorporated herein by reference, disclose a fiber optic array formed of heterogeneous, semi-selective thin films which function as sensing receptor units and are able to detect a variety of different analytes and ligands using spectral recognition patterns. This technology has been applied to a vapor-sensing system which utilizes arrays of polymer-dye combinations which coat the ends of select optical fibers in a fiber optic bundle. These developments are further described in Dickinson, et al, Nature 382:697 (1996) and White, et al, Anal. Chem. 68:2191 (1996).
An innovative feature of the four previously referenced patents to Walt, et al., was the placement of multiple chemical functionalities at the end of a single optical fiber bundle sensor. This configuration yielded an analytic chemistry sensor that could be remotely monitored via the typically small bundle. The drawback, however, was the difficulty in applying the various chemistries associated with the chemical functionalities at the sensor's end; the functionalities were built on the sensor's end in a step-wise serial fashion. This was a slow process, and in practice, only tens of functionalities could be applied.
U.S. patent application Ser. No. 08/818,199 to Walt, et al, the teachings of which are incorporated herein by this reference, discloses the use of dye infiltrated polymer microspheres as a substitute for polymer-dye coating layers in optical fiber array sensors. With this approach, a fiber optic bundle serves as a substrate for dye-polymer microsphere array which contains a variety of microsphere bead sensors having different chemical and optical responses to the presence of target analytes. One innovative feature of this invention is in providing for a bead-based analytic chemistry system in which beads or microspheres carrying different chemical functionalities may be mixed together while retaining the ability to identify the functionality of each bead using an optically interrogatable encoding scheme. Additionally, this invention provides for an optical fiber bundle sensor in which the separate beads or microspheres may be optically coupled to discrete fibers or groups of fibers within the bundle. While the innovative features of this invention have separate applications, when implemented together, the invention provides for an optical fiber sensor that can support large numbers, thousands or more, of separate chemical sensor elements, which can be incorporated into a chemical sensor array and chemical analysis system. This approach provides for rapid fabrication and assembly of individual sensors and complex sensor arrays containing a multitude of discrete sensor types. The method also provides for a high degree of reproducibility and conformity within a batch of sensors and sensor arrays. Additional advantages are realized due to the ultrafine sizing available in microspheres. The overall size of the sensor array can be substantially reduced to submillimeter scale. This reduction in scale is particularly advantageous for remote sensing arrays.
While the method of applying microsphere sensor elements in chemical sensor arrays as taught in U.S. patent application Ser. No. 08/818,199 to Walt, et al, has many innovative features, this method has certain limitations. The method requires a complex multi-step bead encoding process to identify the type and location of bead subpopulations used in the sensor array. Beads are encoded by employing combinations of fluorescent dyes in varying ratio. The choice of encoding dyes is limited to those dyes which emit light at different wavelengths upon exposure to excitation light energy. While combinations of dyes in different ratios provide for encoding subpopulations of beads, the number of dye ratios available for encoding beads with a given dye pair or combination is significantly limited due to crowding the emission spectrum from peak overlap. In addition, a separate reporting dye is necessary for obtaining a unique characteristic optical response signature for a target analyte. Thus, the encoding dye choice is further limited by selecting dyes whose emission wavelengths do not overlap or interfere with the reporting dye which is uniquely responsive to the presence of an analyte.
Another limiting feature of this invention is that the process of encoding beads requires a series of measurements which calibrate and train the sensors and the sensor array. Encoding is initially accomplished by first illuminating the beads with excitation light energy and monitoring and recording the type and location of the specific bead subpopulation within the sensor array having a given encoding dye ratio. Next, the array is exposed to an analyte while illuminating the array with excitation light energy in the presence of a reporter dye. Those beads which are responsive to the analyte in the presence of the reporter dye are monitored and mapped on the sensor array. In addition, the characteristic optical response signature is stored in a library. This step is repeated for each analyte of interest in combination with a reporter dye. Once all bead subpopulations are encoded and their response characteristics monitored and recorded, the entire sensor array must be decoded for each analyte by indexing each sensor element with the stored optical response signature for each analyte. This process of decoding individual subpopulations of beads may be require additional steps when a large number of subpopulations are deployed in the array, thereby increasing the training time required for each array.
Other alternative approaches to bead encoding, utilizing molecular tagging, capillary gas chromatography and electron capture detection have been disclosed by Still, et al, Acc. Chem. Res. 29:155 (1996). However, such methods are limited in scope and have been applied only to a narrow class of bead materials having specific chemical functionality and molecular tags which are readily analyzable.